1. Field of the Invention
The present invention relates to an image pickup lens for use with a film-based camera, a digital still camera, and a digital video camera, and more particularly to a fisheye lens system and a fisheye zoom lens.
2. Description of the Related Art
A general fisheye lens is a single focus lens, such as a diagonal fisheye having an angle of field of about 180° in a diagonal direction of an image sensor and a circular fisheye having an angle of field of about 180° in all directions. Frequently used image sensors are a full-size image sensor (having a screen size of 36×24 mm and a diagonal length of 43.2 mm), an APS-H image sensor (having a screen size of 28.1×18.7 mm and a diagonal length of 33.8 mm), an APS-C image sensor (having a screen size of 22.5×15.0 mm and a diagonal length of 27.0 mm). Zoom lenses having a diagonal fisheye lens range having an angle of field of about 180° are also proposed (Japanese Patent Laid-Open No. 2007-94371 and Japanese Patent No. 3646717).
Japanese Patent Laid-Open No. 2007-94371 discloses a zoom lens configured to provide a diagonal fisheye at the shortest focal length for the APS-C image sensor.
Japanese Patent No. 3646717 discloses a zoom lens configured to provide a diagonal fisheye in a camera having a plurality of screen sizes.
The angle of field of the fisheye lens has a finite value up to θ of about 180°, and its largest image height is proportional to the focal length. A fisheye lens that is commonly used for a lens exchange type camera having a variety of image sensor sizes cannot achieve a angle of field of 180° or can achieve the angle of field of 180° in only a few directions (such as diagonal directions) although it is called as a fisheye lens if the image sensor size used for the camera is small.
A term “zooming” in a fisheye lens means a change of an area in which the angle of field of 180° is achieved as the focal length f changes. Therefore, in zooming of a camera that uses a single image sensor, the fisheye zoom lens intends to form an image as an area that achieves an angle of field of 180° changes in the screen. When an image pickup area is within the angle of field of 180°, a cutout of an angle of field starts and a so-called telescopic phenomenon occurs. A circular fisheye refers to a state that achieves 180° in the entire circumferential angle of field, and a diagonal fisheye refers to a state that achieves an angle of field of 180° only in the screen's diagonal areas.
When cameras having image pickup elements having different sizes are used, the zoom ratio means an image pickup element range that can achieve an angle of field of 180°. It is thus highly likely that the camera having a smaller image pickup element can achieve the angle of field of 180°.
When the angle of field of 180° is achieved, a “wide angle end” and a “telephoto end” at a zoom position are useless, and this specification uses terms of the “shortest focal length side or shortest focal length” and the “longest focal length side or longest focal length”.
In this situation, the above prior art disclosed in Japanese Patent Laid-Open No. 2007-94371 has a problem in that the circular fisheye is unavailable even with a full-size image sensor due to an insufficient small focal length on the shortest focal length side. In addition, the above prior art disclosed in Japanese Patent No. 3646717 has a problem in that a fisheye zoom lens is unavailable that satisfies a circular fisheye to a diagonal fisheye with a full-size image sensor because of an insufficient magnification varying ratio.